Our results obtained in the period between February 1977 and January 1978 describe the extensive properties of adrenal steroid hydroxylase. In particular, the antiferromagnetic exchange coupling constant was determined to be J equals 497 per cm. Human reduced adrenodoxin displayed an EPR signal with g equals 1.940 and g equals 2.027 and its content in human adrenal mitochondria was found to be 0.65 nmol/mg protein.